This application is related to co-pending U.S. patent application Ser. No. 09/666,582 entitled xe2x80x9cPower Delivery Circuit With Boost For Energetic Starting In A Pulsed Charge Starter/Alternator Systemxe2x80x9d filed Sep. 21, 2000 and U.S. patent application Ser. No. 09/667,087 entitled xe2x80x9cPulsed Charge Power Delivery Circuitxe2x80x9d filed Sep. 21, 2000 both of which are incorporated herein by reference and commonly owned by the assignee of the present invention.
The present invention relates to electrical systems for motor vehicles and more particularly, to a pulsed charge control system for a vehicle electrical system having a combined starter/alternator.
Combined starter/alternators such as those known in the art are disclosed in U.S. Pat. Nos. 4,720,638; 4,916,345; 5,001,412; 5,097,140 and 5,469,820. A combined starter/alternator as used in a motor vehicle can be used as a motor to crank and start the engine as well as a generator to provide electrical power to the vehicle electrical systems.
When operated as a starter motor, the starter/alternator is supplied with current from the battery and outputs a high torque to crank the engine until the engine is started. When used as a generator, the running engine is coupled to the starter/alternator which, in turn, produces a three-phase output that is rectified to a steady state DC output that is used to maintain the charge on the vehicle energy storage device to meet the vehicle electrical load requirements.
When operating as a generator, the efficiency of the starter/alternator is defined as the ratio of the input power to the output power. Such generators have both fixed and variable losses. Some of these losses are associated with the switching circuitry such as the inverter used to rectify the output of the generator. There are three primary classifications of losses: mechanical losses, electrical losses, and magnetic losses. Mechanical losses are primarily due to the rotation of the rotor and include bearing friction loss and windage loss. Magnetic losses include eddy current-current loss and hysteresis loss. All of these losses can be grouped into two categories, namely, fixed losses and variable losses. Fixed losses are losses that do not change with load when the machine is operating at a known speed. Therefore, rotational losses are one part of fixed losses. In the case of a wound machine where the field current is required to set up the required flux in the machine, the power supplied to the field lining is also considered a fixed loss. On the other hand, variable losses are losses that vary with the load current. All of the copper losses are included in this category. Since the losses associated with the starter/alternator when operating as a generator are only partially load dependent, the system exhibits low efficiencies at low power levels. At higher output power levels, the output power rises relative to the losses and correspondingly, the system efficiency rises as well. This continues up to a peak efficiency torque point wherefore additional torque input into the generator does not result in a significant increase in power output and, hence, the efficiency falls off.
It has been found that operating a starter/alternator at a continuous power output approximately equal to the vehicle electrical load demand such as 500W is a low efficiency output for a typical starter/alternator. Accordingly, there is a need for a starter/alternator control system having increased efficiency.
Accordingly, it is an object of the invention to provide a starter/alternator control system for a motor vehicle having a high efficiency. According to the present invention, the foregoing and other objects are attained by a system and method of charging an energy storage device in a vehicle including an engine coupled to a combined starter/alternator and an energy storage device connected to said starter/alternator. The energy storage device provides electrical power to the vehicle. The method comprises the steps of determining a peak efficiency torque value of the starter/alternator when operating as an electrical generator, and intermittently operating the starter/alternator for a predetermined period of time as an electrical generator at approximately the peak efficiency torque value. In this way, the output of the starter/generator is substantially greater than an average power load value of the vehicle for a predetermined period of time, but the average power output of the starter/generator is approximately equal to the average power load requirements of the vehicle. In one aspect of the invention, the engine output torque is increased by the peak efficiency torque value associated with the starter/alternator, and a switching circuit is periodically activated to maintain the desired charge on the energy storage device.
An advantage of the present invention is that it has an increased system efficiency as compared to conventional starter/alternator control systems. Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.